I don't think that there is a specific time at which you can start training in Aikido
Aikido is considered to be one of the hardest martial arts to master.
Aikido,"the way of divine harmony", was developed from Zen philosophy in the belief that the center of mediation and the source of mental strength or power, ki, is centered in the body, about an inch above the navel.
Aikido is essentially noncombative, emphasizing throwing and joint techniques over striking and kicking techniques. Aikido students are taught to react to an attack by throwing and subduing the aggressor by applying pressure to joints or vital points causing pain and submission.
Founded in 1938 by Ueshiba Morihei, a martial artists who had studied daito-ryu aiki-ju-jitsu. He started teaching his new ‘way of divine harmony’ which was dubbed Aikido to promote a higher level of martial artist for 20th Century Japan.
The techniques Ueshiba created emphasized throwing and ensnaring by the use of circular movements. Evasion techniques are used to avoid conflict; the art is reminiscent of the internal schools of Chinese martial arts. Ueshiba placed great emphasis upon the harmony of mind, body and spirit and discouraged the sporting, competitive aspects of training. Since Aikido is an entirely defensive art the notion of practicing free form sparring was unacceptable, and morally repugnant.
Ueshiba died in 1969 and the development of Aikido was taken over by his son, Kisshomaru Ueshiba. Aikikai is the name give to his central dojo in the Shinjuku district of Tokyo where classes are conducted 7 days a week.
The teaching of the Daito-ryu aiki-jujitsu, the roots of Aikido, continue today, under the guidance of Tokimune Takeda in Tokyo Japan. Among Ueshiba’s senior disciples, in addition to his son, are Koichi Tohei, Gozo Shioda, Minoru Mochizuki and Kenji Tomiki.
Several of his most prominent students broke from Ueshiba’s teachings to develop their own style of Aikido.
Tomiki, who had also studied judo before joining Ueshiba, decided to create his own style of sport Aikido. It combines many special techniques that derive from judo and is taught in many Japanese universities as well as other countries.
Shioda’s yoshin style is combat-oriented and closely resembles the classical sect of aiki-jujitsu, though it’s spiritual purpose is like Ueshiba’s Aikido. This system is too popular and taught throughout the world.
WBB